I'm going to use one of those prebuilt relay boards to turn some audio amplifiers on/off. I'll be switching between the DC voltage out of the power supply and the amplifier. The Arduino will be in the same case as the power supply and amps. Is optical isolation needed in this instance or will a simple transistor be sufficient?

Thanks. But I just realized that I can use a pwoer fet since it will be DC? Is that correct? Something like this: http://arduino-direct.com/sunshop/index.php?l=product_detail&p=60Do they make those without optical isolation? I can't find any pre-built ones.

But I just realized that I can use a power fet since it will be DC? Is that correct?

Someone recommended the FQP50N06 recently. I've never used it, but it's good up to 50 Amps, so it should be good for almost anything.

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I'm going to use one of those prebuilt relay boards...

Is optical isolation needed...?

FYI - Mechanical relays are isolated (mechanically rather than optically). There is no connection between the coil and the contacts. Of course, you can use a common ground and/or a common power supply, so the two sides don't have to be isolated in your circuit.

And while we're on the subject, transformers also provied isolation (electromagnetically). But transformers don't pass DC.

There is inductive kickback when you de-energize the coil of a relay. A diode on the coil is supposed to prevent damage to the arduino but I wasn't sure if this was enough or not. I think I will go with the power fet method. That looks like a pretty nice board for $16 if I can find someone who has one in stock.

I ended up going with this one: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270810446882

I could have made one without optical isolation as stated earlier, but I figured this prebuiilt board would look cleaner. I will be controlling a 2x100 watt amp http://hifimediy.com/?wpsc-product=hifimediy-t1-no-volume-pot-sta510a-2100w8ohm-amplifier at 36 volts off each transistor. So in theory, I could be pulling 5.56 amps on each transistor? Do you think I should put a heat sync on the transistors?